An ecolect is a dialect variant that develops on the scale of a household. Also see oikolect.

Basically, if a few people live in close quarters for years, they pick up expressions, favorite words, and sentence-making habits from their housemates. Idioms unique to a household can develop.

Some examples:

• The first thing I'd think of is the Incandenza family from David Foster Wallace's _{may he rest in peace} Infinite Jest. We get a submarine view of the Incandenzas, ecolect and all, and they have a rich one: Avril's a Canuck and Hal is a linguistics genius. So the Incandenza ecolect is a lot of esoterically phrased verbiage and random odd-seeming slang. howling fantods, prandial, et cetera
• With my family, referring to "an attack of SIDS" means someone has made a contextless comment that has weirded up the conversation significantly. It says a lot about my family that we have a special phrase for this.
• Can ecolects develop among friends? If so, killing dinosaurs is an idiom of one ecolect I speak, and Marwoodian is another.

Those are really sloppy laypersyn's examples, probably more in the idiom-centered vein of family slang. I'm sure I could do an exhaustive survey of sentence-construction patterns, actually learn something about what I'm talking about, make a heavy black bookful of big slopy graphs with important-looking cranium-resembling peaks and feel extremely special about myself, get a diploma, get a life, blagh blagh blagh

Just another thing I learned from Joey Comeau. He's at untoward.livejournal.com and his blog is high entertainment. Because I learned this word from Mr. Comeau, I will always associate it with locks, adorable queer faces, and reindeer dragging their hooves wearily through northerly blizzards.